Chapter 2 Chemical and Physical Properties of Sulphur Dioxide and Sulphur Trioxide

Transcription

1 Chapter 2 Chemical and Physical Properties of Sulphur Dioxide and Sulphur Trioxide 2.1 Introduction In order to appreciate the impact of the properties of liquid sulphur dioxide and liquid sulphur trioxide on future technology, it is important that an in-depth analysis of their properties be understood. Though the data given in this chapter are available in literature, the practical application of the remarkable physical as well as chemical properties of sulphur dioxide and sulphur trioxide has been experienced and applied on large scale only recently. The three main features of these two important chemicals are: (a) (b) High solubility of sulphur trioxide in liquid sulphur dioxide Reaction of liquid sulphur trioxide with liquid sulphur in stoichiometric proportions instantaneously to produce sulphur dioxide: S þ 2SO 3 ¼ 3SO 2 (c) Liquefaction of pure sulphur dioxide at room temperatures under moderate pressures of 5 6 kg/cm 2 (Please see Fig. 2.1). The present sulphonation techniques involves sulphonating agents such as sulphuric acid, 25 % oleum, 65 % oleum and sulphur trioxide. The technique involves high temperature reactions due to exothermic nature of sulphonation. The current techniques of sulphonation require elaborate chilling and cooling systems. Sulphonating processes currently used are generally batch operations and hence requires a battery of reactors having varying time cycles. The Author(s) 2016 N.G. Ashar, Advances in Sulphonation Techniques, SpringerBriefs in Applied Sciences and Technology, DOI / _2 9

2 10 2 Chemical and Physical Properties Fig. 2.1 Vaporization curves for sulphur dioxide PRESSURE OF SO 2. BAR SO Concentration (Vol. - %) Curve for 100% SO 2 shows relationship of boiling point to equilibrium vapour pressure Curve for impure SO 2 mixture shows dependence of dewpoint on SO 2 partial pressure DEWPOINT BOILING POINT C 2.2 Sulphur Dioxide Physical Properties Sulphur dioxide SO 2 is a colourless, non-inflammable, toxic gas with a characteristic pungent smell and acidic taste. Table Vaporisation of SO 2 It is important to analyse the physical property of condensation points at various pressures and concentrations of SO 2.

3 2.3 Vaporisation of SO 2 11 Table 2.1 Physical properties of sulphur dioxide Property Value Molecular weight Melting point (1013 mb) 75.5 C Latent heat of fusion (at m.p) J/g Dynamic viscosity at 0 C 368 Pa/s Density at 10 C 1.46 g/cm 3 Critical density g/cm 3 Critical pressure 78.8 bar Critical temperature C Boiling point (1013 mb) 10 C Latent heat of vaporization (at b.p.) 402 J/g Standard density at 0 C (1012mb) 2.93 kg/m 3 Density relative to air (0 C, 1013mb) Molar volume (0 C, 1013 mb) 21.9 l/mol Standard enthalpy of formation kcal/mol 4636 J/g Specific heat, Cp (1013 mb) 0 C 586 K/(kg K) 100 CC 662 J/(kg K) 300 C 754 J/(kg K) 500 C 816 K/(kg K) Cp/Cv (15 C, 1013 mb) 1.29 It can be observed from the attached Fig. 1.2 that for 100 % liquid SO 2 moderate pressures are required to liquefy SO 2 at ambient temperatures between 30 and 40 deg C. 2.4 The Solubility of SO 2 in Sulphuric Acid The solubility of sulphur dioxide in Sulphuric acid (see Fig. 1.3) rises in proportion to the SO 2 partial pressure in good conformity with Henry s law and is increased by lowering the temperature, as represented graphically in Fig In the solution, sulphur dioxide is present mainly as SO 2 molecules, but Raman spectroscopy confirms the presence in minor proportions of the species HSO 3,S 2 O 5 and H 2 SO 3. The last of these, sulphurous acid (the anhydride of which is sulphur dioxide), exists only in aqueous solution. Aqueous solution of alkaline compounds will absorb much more sulphur dioxide than pure water (Please see Fig. 2.2) because of the formation of hydrogen sulphite (bisulphite) and sulphite ions.

4 12 2 Chemical and Physical Properties Fig. 2.2 Solubility of sulphur dioxide in water 2.5 Solubility of Sulphur Dioxide in Water It can be observed from Fig. 2.2 that the solubility of sulphur dioxide in g/kg H 2 O increases with pressure and reduces with temperature. This property is of importance in industrial applications in scrubbling of sulphur dioxide in tail gases. 2.6 Chemical Properties of Sulphur Dioxide Sulphur dioxide is very stable; thermal dissociation becomes significant only above 2,000 C. It can be decomposed by shock waves, irradiation with ultraviolet or X-rays, or by electric discharges The reaction of sulphur dioxide with oxygen to form sulphur trioxide is industrially the most significant of all its reactions because of its importance in sulphuric acid production. In the gas phase, it will only take place at elevated temperatures and, for a satisfactory yield of sulphur trioxide; it requires the presence of a catalyst. In aqueous solution, sulphur dioxide is oxidized to sulphuric acid at low temperatures by air in the presence of activated coke or nitrous gases or by oxidizing agents like hydrogen peroxide.

5 2.6 Chemical Properties of Sulphur Dioxide 13 The reduction of sulphur dioxide with hydrogen, carbon or carbon compounds such as methane or carbon monoxide is also of industrial interest. These reactions require high temperatures or catalysts or both. They result in mixtures of elemental sulphur with hydrogen sulphide. If carbon or a carbon compound has been used as the reducing agent, carbon-containing species such as carbon dioxide, carbonyl sulphide and carbon disulphide will be formed as well. Sulphur dioxide will normally oxidize metals at elevated temperatures, simultaneously forming metal sulphides and oxides. Liquid sulphur dioxide is a relatively efficient solvent with some water-like properties. Polar inorganic compounds are usually insoluble or only sparingly soluble in liquid sulphur dioxide, whereas covalent inorganic and organic compounds are often dissolved, mostly forming stable solutions. The fact that aromatic hydrocarbons will dissolve more readily than aliphatics in sulphur dioxide is exploited on an industrial scale for the extraction of aromatics from crude oil according to the Edeleanu process. 2.7 Physical Properties of Sulphur Trioxide Sulphur trioxide is produced by catalytic oxidation of sulphur dioxide in concentrations of % in gaseous form. To produce pure sulphur trioxide the plant gases are passed through oleum towers to produce % free SO 3 oleums. These oleums are boiled in steam heated or gas heated heat exchangers to produce pure sulphur trioxide. This is then sent to condensers to produce liquid sulphur trioxide. 2.8 General Properties of Liquid Sulphur Trioxide Empirical formula SO 3 Molecular wt. of monomer Boiling point 44.8 C (112.6 F) Density (20 C) Specific heat (cal/g at C) 0.77 Heat of dilution (cal/g) 504 Critical temperature C (424.9 F) Critical pressure 83.8 atm Critical density g/ml van der Waal s constants a = 2105 b = 0.964

6 14 2 Chemical and Physical Properties 2.9 Properties of Liquid Sulphur Trioxide See Fig Fig. 2.3 Properties of different molecular forms of liquid sulphur trioxide

7 2.10 Viscosity of Liquid Sulphur Trioxide 15 Fig. 2.4 Viscosity of liquid sulphur trioxide 2.10 Viscosity of Liquid Sulphur Trioxide See Fig Specific Gravity of Sulphur Trioxide See Fig Vapour Pressure of Liquid Sulphur Trioxide See Fig. 2.6.

8 16 2 Chemical and Physical Properties Fig. 2.5 Properties of sulphur trioxide 2.13 Molar Heat Capacity of Liquid Sulphur Trioxide See Fig Vaporisation Curves for Sulphur Dioxide See Fig Enthalpy of Sulphur Trioxide Gas See Fig. 2.8.

9 2.16 Chemical Properties of Sulphur Trioxide 17 Fig. 2.6 Properties of sulphur trioxide 2.16 Chemical Properties of Sulphur Trioxide Commercially Sulphur Trioxide Is Produced by Converting % SO 2 by Catalytic Conversion at Temperatures Between C in Multipass Converter of Sulphuric Acid Plant This is then further reacted with water to form Sulphuric acid by the equation H 2 S 2 O 7 þ H 2 O! 2H 2 SO 4 It is important to note that reaction of sulphur trioxide gas with water would form micron size droplet and cannot be absorbed to form H 2 SO 4.

10 18 2 Chemical and Physical Properties Fig. 2.7 Properties of sulphur trioxide Formation of Sulphuric acid from SO 3 gas is exothermic and the absorbing H 2 SO 4 in the absorption towers need to be cooled to maintain efficiency of absorption One of the Special Chemical Properties of SO 3 Which Has Been Safer but not Explored till date S þ 2SO 3liq! 3SO 2gas DH ¼ 74:3 kcal=g mole DF ¼ 36:71 kcal=g mole Since the free energy change is large and negative, the reaction is almost instantenous. In addition, the reaction generates one additional mole in gaseous form, so there is a pressure increase.

11 2.18 Sulphur Trioxide Is a Strong Sulphonating Agent 19 Fig. 2.8 Properties of sulphur trioxide 2.18 Sulphur Trioxide Is a Strong Sulphonating Agent for Difficult, Organic and Inorganic Chemicals Treatment of Sulphuric Acid Plant Tail Gas from Final Absorption Tower The tail gases of sulphuric acid contact plants consist chiefly of nitrogen and residual oxygen. They also contain sulphur dioxide in low concentrations which depend on the conversion efficiency attained in the conversion stages. The content of gaseous sulphur trioxide and sulphuric acid is essentially a function of the temperature and concentration of the irrigation acid in the final absorber. Under unfavourable operating conditions, as, for example, when the sulphur dioxide-containing converter feed gases are inadequately dried or contain hydrocarbons, sulphuric acid mists can be formed which are not removed in the absorption system, even when the concentration and temperature of the absorber acid are at their optimum values. The safest way of removing these acid mists is using a candle type demister. However, this is not very effective in removing excessive sulphur trioxide concentrations, which may result from poor acid distribution in the absorber.

12